From Sonic Stadium: “Roger Craig Smith has revealed that he chose to step away from voicing Sonic the Hedgehog, and was not a result of SEGA decision-making.

In an interview with Voices from Krypton, the voice actor whose other roles have ranged from Batman to Assassins Creeds Ezio Auditore to Bugsnaxs Snorpy Fizzlebean said that he felt he had naturally given as much as he could to SEGAs blue-blur mascot. The timing simply felt right for him to step away and explore new roles, he suggested.”

Source: N4G PC Roger Craig Smith Reveals Sonic Departure Was His Decision, "It's Time To Be Done"